Question

# A decapeptide (MW 796) on complete hydrolysis gives glycine (MW 75), alanine and phenylalanine. Glycine contributes 47.0% of the total weight of the hydrolyzed products. The number of glycine units present in the decapeptide is:a.) 6b.) 5c.) 7d.) None of these.

Hint: Peptide is a small chain of amino acids. The amino acids in the peptide chain are connected to one another with a bond called peptide bond.
Decapeptide means the peptide contains ten amino acids in its chain and nine peptide bonds are going to hold these ten amino acids.

In the question it is given that the peptide is decapeptide and molecular weight of the decapeptide is 796.
On complete hydrolysis of the decapeptide it forms glycine, alanine and phenylalanine.
We know that nine peptide bonds are present in decapeptides.
The nine peptide bonds require nine moles of water to do the complete hydrolysis.
Means nine water molecules are going to be added to the decapeptide during the hydrolysis.
Total weight of the water added = 9$\times$ 18 = 162
Therefore total weight of the hydrolyzed product = 796 + 162 = 958
In the question it is given that glycine contributes 47% of the total weight of the hydrolyzed products.
Total weight of glycine in product = $\dfrac{958\times 47}{100}=450g$
Molecular weight of the glycine is 75.
Hence the number of glycine formed during the hydrolysis = $\dfrac{450}{75}=6$
Therefore the decapeptide on hydrolysis produces 6 glycine residues.
So, the correct answer is “Option A”.

Note: A lot of peptides combine and form peptide chains. So many peptide chains combine and form proteins. The basic unit of the protein is an amino acid. So, amino acids are called as building blocks of proteins.